My Son Had Excision Of A Pilondial Cyst...actually This Was

My son had excision of a pilondial cyst...actually this was his 3rd surgery. The surgery was 3 weeks ago and he started having weakness, pain and some paresthesia in his left leg only. We had ultrasound today to r/o dvt which was negative for dvt. Any ideas on what could be causing these symptoms. He is 16 years old.